Direct positive photographic elements and processes for preparing direct positive silver halide emulsions

ABSTRACT

This invention relates to direct-positive, silver halide, photographic elements. In one aspect, at least one layer of said element contains nitron. In another aspect, an overlayer containing light-insensitive silver chloride is used to provide more uniform photographic characteristics in used developers. In still another aspect, improved direct-positive silver halide grains are prepared by reducing a portion of the water-soluble silver salt before admixture with the water-soluble halide to precipitate the silver halide grains.

United States Patent [191 Barbier et al.

[ 1 June 28, 1974 DIRECT-POSITIVE PHOTOGRAPHIC ELEMENTSAND PROCESSES FOR PREPARING DIRECT-POSITIVE SILVER HALIDE EMULSIONS [75] Inventors: Jean-France L. P. Barbier; Guy

Renard, both of Vincennes, France [73] Assignee: Eastman Kodak Company,

Rochester, NY.

[22] Filed: Feb. 3, 1972 [21] App]. No.: 223,372

Related US. Application Data 162] Division of Ser. No. 874,393, Nov. 5, 1969, Pat. No.

52 us. c|.... 96/64, 96/108 51 rm. Cl G03c s/24 5 Field Of Search 9 /107, 108, 94, 68

[56] References Cited UNITED STATES'PATENTS 2,401,051 5/1946 Crouse et a1.. 96/94 R 7/1955 Weyde 96/94 2,875,052 2/1959 Weyde 96/68 3,367,778 2/1968 Berriman 96/107 3,477,852 1 H1969 Allentoff et al..,..

3,531,290 9/1970 Litzerman 96/107 OTHER PUBLICATIONS APC Stasiw S.N. 418502 5/4/43.

Primary EvaminerRonald H. Smith Assistant Eranziner-John L; Goodrow Attorney, Agent, or Firm-Carl 0. Thomas [57 ABSTRACT 7 Claims, No Drawings 1 DIRECT POSITIVE PI-IOTOGRAPIIIC ELEMENTS AND PROCESSES FOR PREPARING DIRECT-POSITIVE SILVER HALIDE EMULSIONS This is a divisionof application Ser. No. 874,393, filed Nov. 5, 1969, now US. Pat. No. 3,679,424, issued July 25, 1972.

This invention relates to direct-positive, fogged silver halide emulsions. In one aspect, this invention relates to direct-positive silver halide elements comprising at least one layer which contains nitron; In another aspect, this invention relates to a direct-positive photographic element which comprises at least one directpositive silver halide emulsion layer and at least an overcoat layer and/or an interlayer in said element which containsnitron. In still another aspect, this in vention relates to a process for fogging direct-positive emulsions andthe'produc'ts produced thereby.

It is known in theprior art to make direct-positive,

fogged silver halide emulsions. Emulsions of this type' can be used in photographic elements to provide positive images upon exposure and chemical development. In many of the prior directpositive elements, image characteristics upon'developm ent were highly dependent upon development conditions, length of use of the developersolutions, etc. The formation of a yellow fog is especially prevalent when the developing solution has been extensively used. Density and contrast changes also occur with used developers as opposed to fresh developing solutions. Therefore, improved directpositivephotographic elements and processes for making direct-positiveemulsions would be desirable to provide direct-positive elements which demonstrate better image properties under a wide variety of exposure and 7 tainingnitron can be processed after exposure in either fresh developer or extensively used developer to obtain substantially uniform sensitometric characteristics in the photographic element. In another aspect, we have found that a silver chloride overlayerwhich preferably contains an excess of chloride ions on the direct.- positive element provides more uniform image propertieswithout substantial effects due to length of useof the developer, such as occurs when bromine ions build up in the developer. In still another aspect of this invention, we have found that a portion of the silver nitrate used in the precipitation of the direct-positive silver halide can be reduced with a reduction agent such as thiourea dioxide, stannous chloride and the like to provide improved direct-positive emulsions.

In a preferred embodiment, the direct-positive emul-. sions used in combination with layers which contain nitron are direct-positiveblue-sensitive, chemically fogged silver halide emulsions.

In another preferred embodiment, the direct-positive emulsions comprise halogen-accepting compounds.

In another preferred embodiment, the direct-positive emulsions comprise electron-conducting compounds which are sometimes referred to as desensitizers.

In one preferred embodiment of this invention, the direct-positive photographic elements comprise layers which contain nitron. Generally, the nitron is utilized at a concentration of about 100 milligrams to about 2 grams per mole of silver halide in the element and preferably from about 300 milligrams to about 1 gram. Nitron is generally known in the trade to be 1,4-diphenyl- 3,5-endo-anilino-4,5-dihydrol ,2,4-triazole or 3,5,6- triphenyl-2,3,5,6-tetraazabicyclo[ 2. l. l ]hexl -ene.

The preferred direct-positive silver halide emulsions of this invention are blue-sensitive. It is understood that blue-sensitive means that the direct-positive composition will provide a reversal image when exposed with light in the 350- to SOO-millimicron range of the electromagnetic spectrum. The silver halide compositions can also be spectrally sensitized so as to form reversal images when exposed in other regions of the spectrum such as the red and green regions. However, they all have the property of being capable of forming a reversal image when exposed with light in the blue region of the spectrum. Generally, these emulsions have high photographic speed compared to compositions such as conventional Hershel reversal emulsions.

Typical -blue-sensitive-direct-positive silver halide emulsions which can be used in combination with nitronin photographic elements with improved properties include those disclosed in Berriman, U.S. Pat. No.

3,367,778 issued Feb. 6, 1968; llllingsworth, Belgian Patent Nos. 695,355 through 695,366, all of which were granted Sept. 1 I, 1967; and Allentoff and Fogler,

- Belgian Pat. No. 689,233 granted Jan. 13, I967. In one embodiment, the silver halide emulsions can comprise silver halide grain centers which promote the deposition of photolytic silver. In one embodiment of the invention, the sites for deposition of photolytic silver are provided by reducing a portion of the water-soluble silver salt such as silver nitrate before admixture in the reaction vessel with the water-soluble halide such as, for example, potassium bromide. Reduction of the silver nitrate can take place with any common .reducing agent such as, for example, stannous chloride, thiourea dioxide, formalin, alkaline arsenite and the like. In this embodiment, it is also advantageous to add small amounts of polyvinyl pyrrolidone to the silver nitrate before reduction to obtain high contrast and maximum density with a small silver halide coverage ratio.

Typical direct-positive silver halide compositions which can be characterized by the above definitions and which are useful in this invention are: l) emulsions .comprising silver halide grains having internal centers which promote the deposition-of photolytic silver and an outer region or shell of a fogged. insoluble silver salt and preferably a halogen-conducting compound in said emulsion or 2) an emulsion which comprises fogged silver halide grains and an organic compound which accepts electrons, said grains being such that a test portion thereof, when coated as a photographic silver halide emulsion on a support to give a maximum density of at least about 0.5 upon processing for 5 minutes at about 68 F. in Developer A (formula at end of specification), hasa maximum density which is at least about 30 percent greater than the maximum density of an identical coated test portion which is processed for 6 minutes at about 68 F. in Developer A after being bleached for about minutes at about 68 F. in a bleach composition of:

This invention can be practiced with direct-positive emulsions of the type in which a silver halide grain has a water-insoluble silver salt center and an outer shell composed of a fogged water-insoluble silver salt that develops to silver without exposure. These emulsions can be prepared in various ways, such as those described in Berriman, US. Pat. No. 3,367,778 issued Feb. 6, 1968. For example, the shell of the grains in such emulsions may be prepared by precipitating over the core grains a light-sensitive, water-insoluble silver salt that can be fogged and which fog is removable by bleaching. The shell is of sufficient thickness to prevent access of the developer used in processing the emulsions of the invention to the core. The silver salt shell is surface fogged to make it developable to metallic silver with conventional surface image developing compositions. The silver salt of the shell is sufficiently fogged to produce a density of least about 0.5 when developed for 6 minutes at 68 F. in Developer B below when the emulsion is coated at a silver coverage of 100 mg. per square foot. Such fogging can be effected by chemically sensitizing to fog with the sensitizing agents described for chemically sensitizing the core emulsion, high-intensity light and the like fogging means wellknown to those skilled in the art. While the core need not be sensitized to fog, the shell is fogged. Fogging by means of a reduction sensitizer, a noble metal salt such as gold salt plus a reduction sensitizer, a sulfur sensitizer, high pH and low pAg silver halide precipitating conditions, and the like can be suitably utilized. The shell portion of the subject grains can also be coated prior to fogging.

Developer B I v Before the shell of water-insoluble silver salt is added to the silver salt core, the core emulsion is first chemically or physically treated by methods previously described in the prior art to product centers which promote the deposition of photolytic silver, i.e., latent image nucleating centers. Such centers can be obtained by various techniques as described in the Berriman patent referred to above. Silver salt cores containing centers attributable to a metal of Group VI of the Periodic Table, e.g., palladium, iridium or platinum and the like, are especially useful since these centers also appear to function as electron acceptors. Chemical sensitization techniques of the type described by Antoine Hautot and Henri Saubenier in Science e! Industries P/zolographiques, Vol. XXVlll, January, 1957, pages 1 to 23, and January, 1957, pages 57 to 65, are particularly useful. Such chemical sensitization includes three major classes, namely, gold or noble metal sensitization, sulfur sensitization, such as by a labile sulfur compound, and reduction sensitization, e.g., treatment of the silver halide with a strong reducing agent which introduces small specks of metallic silver into the silver salt crystal or grain.

In another embodiment, the silver halide emulsions can comprise silver halide grains having centers which promote the deposition of photolytic silver which are either sufficiently small or sufficiently buried within the crystal as to be not accessible to initiate development to a visible image. Silver halide grains of this type can be provided by either using very low concentrations of the sensitizing agent throughout the precipitation or adding the sensitizing agent to the precipitation medium during the initial part of the precipitation whereby the concentration of the sensitizing agent will be lowered significantly by occlusion of the agent in the grains so that continued precipitation would result in lowered concentration of centers for promoting deposition of photolytic silver in the outer regions of each grain.

The practice of this invention is particularly suitable for high-speed direct-positive emulsions comprising fogged silver halide grains and a compound which accepts electrons, as described and claimed in lllingsworth, Pat. application Ser. No. 619,909, now US Pat. No. 3,501,306 issued Mar. 17, 1970, and titled Photographic Reversal Materials Ill." The fogged silver halide grains of such emulsions are such that a test portion thereof, when coated as'a photographic silver halide emulsion on'a support to give a maximum density of at least about 1 upon processing for 6 minutes at about 68 F. in Developer A, has a maximum density which is at least about 30 percent greater than the maximum density of an identical coated test portion which is processed for 6 minutes at about 68 F. in Developer A after being bleached for about 10 minutes at about 68 F. in a bleach composition of:

potassium cyanide 50 mg. acetic acid (glacial) 3.47 cc. sodium acetate 1 1.49 g. potassium bromide 1 19 mg. water to 1 liter The grains of such emulsions will lose at least about 25 percent and generally at least about 40 percent of their. fog when bleached for 10 minutes at 68 F. in a potassium cyanide bleach composition as described herein. This fog loss can be illustrated by coating the silver halide grains as a photographic silver halide emulsion on a support to give a maximum density of at least 1.0 upon processing for 6 minutes at about 68 F. in Developer A and comparing the density of such a coating with an identical coating which is processed for 6 minutes at 68 F. in Developer A after being bleached for about 10 minutes at 68 F. in the potassium cyanide bleach composition. As already indicated, the maximum density of the unbleached coating will be at least 30 percent greater, generally at least 60 percent greater, than the maximum density of the bleached coating.

The silver halides employed in the preparation of the photographic emulsions useful in this invention include any of the photographic silver halides as exemplified by silver chloride, silver bromide, silver bromoiodide, silver chlorobromide, silver chlorobromoiodide, and the like. Emulsion blends, e. g., blends of silver chloride and y silver chlorobromide, can be used. Also, the core of the silver halide grain can be composed of silver halide of different composition than that in the outer shell of the gram. I

Silver halide grains having an average grain size less than about 2.microns, preferably less than about 0.5 micron, give particularly good results. The silver halide grainscan be regular and can be any suitable shape such as cubic or octahedral, as described and claimed in lllingsworth, application Ser. No. 619,948, now US. Pat. No. 3,501,305, issued Mar. 17, l970and titled Direct-Positive Photographic Emulsions l." Such grains advantageously have a rather uniform size frequency distribution, as described and claimed in Photographic Reversal Emulsions II. For example, at least 95 percent, by weight, of the photographic silver halide grains are within about 40 percent, preferably within about 30 percent, of the mean grain size. Average grain size can be determined using conventional methods, e.g., as shown in an article by Trivelli and Smith entitled Empirical Relations Between Sensitometric and Size Frequency Characteristics in Photographic Emulsion Series" in The Photographic Journal, Vol. LXXIX, 1949, pages 330-338, and Methods of Particle-Size Analysis, ASTM Symposium on Light Microscopy, by Loveland, 1953, pages 94-122. The fogged silver halide grains in these direct-positive photographic emulsions of this invention produce a density of at least 0.5 when developed without exposure for 5 minutes at.68 F. in Developer A when such an emulsion is coated at a coverage of 50 to about 500 mg. of silver per square foot of support. The photographic silver halides can be coated at silver coverages in the range of about 50 to 500 milligrams of silver per square foot.

tate and 0.005 molar in acetic acid using a carbon paste of pyrolytic graphite electrode, with the voltammetric half peak potential for the most negative anodic response being designated E In each measurement, the reference electrode can be an aqueous silver silver chloride (saturated potassium chloride) electrode at C. Electrochemical measurements of this type are known in the art and are described. in New Instrumental Methods in Electrochemistt by Delahay, lnterscicncc Publishers, New York. 1954; Polarography, by Kolthoff 1 and Lingane, 2nd Edition. lntezrscience Publishers. New York, New York, 1952; Anallvtica] C hemistrr. 36.

2426 (1964) by Elving; and Analytical Chemistry. 30,

1576 (1958) by Adams. Signs are given according to In preferred embodiments of this invention, electron acceptors and halogenconductors (sometimes referred to as halogen acceptors) are present in the directpositive emulsions; When the grains of the silver halide emulsion are substantially free of internal sites for the deposition of photolytic silver, it is essential that an electron acceptorbe present in the emulsion if it is to be reversed by blue light.

The electron acceptors or halogen conductors which give particularly good results in the practice of this invention can-be characterized in terms of their polarographic halfwave potentials, i.e., their oxidation reduction potentials determined by polarography. The electron acceptors useful herein have an anodic polarographic potential and a cathodic polarographic potential which, when added together, give a positive sum. The halogen conductors useful herein havean anodic polarographic potential less than 0.85 and a cathodic polarographic potential which is more negative than --1 .0. Preferred halogen, conductors have. an anodic polarographic potential less than 0.62 and a cathodic polarographic potential which is more negative than 1 .3. Cathodic measurements can be made with a l X 10 molar solution of the electron acceptor in a solvent, for example, methanol which is 0.05 molar in lithium chloride using a dropping mercury electrode with the polarographic halfwave potential for the most positive cathodic wave being designated E Anodic measurements can be made with 1 X 10 molar aqueous solvent solution, for example, methanolic solutions of the electron acceptor which are 0.05 molar in sodium acelUPAC, Stockholm Convention 1953.

Advantageously, these electron acceptors used herein also provide spectral sensitization such that the ratio of minus blue relative speed to blue relative speed of the emulsion is greater than 7, and preferably greater than 10, when exposed to a tungsten light source through Wratten No. 16 and No. 35 plus 38A filters respectively. Such electron acceptors can be termed spectrally sensitizing electron acceptors, However, electron acceptors can be used which do not spectrally sensitize the emulsion.

An especially useful class of electron acceptors which can be used in the direct-positive photographic silver halide emulsions and processes of this invention are cyanine dyes, such as the imidazo[4,5- b]quinoxaline dyes. Dyes of this class are described in Brooker and Van Lare Belgian Pat. No. 660,253 issued Mar. 15, 196 5. In these dyes, the imidazo[4,5- b]quinoxaline nucleus is attached, through the twocarbon atom thereof, to the methine chain. Typical where A represents the atoms necessary tocomplete an acid heterocyclic nucleus, e.g., rhodanine, 2- thiohydantoin and the like, B represents the atoms necessary to complete a basic nitrogen-containing heterocyclic nucleus, e.g., benzothiazol-e, naphthothiazole,

benzoxazole and the like, each L represents a methine linkage, e.g.,

and n is an integer from O to 2, i.e., 0, l or 2. Typical halogen-conducting compounds are disclosed in Wise, Belgian Pat. No. 695,361 granted Sept. 11, 1967.

in the preparation of the above photographic emulsions, the electron acceptors, halogen conductors, bro mide and iodide salt are advantageously incorporated in the washed, finished silver halide emulsion and should, of course, be uniformly distributed throughout the emulsion. The methods of incorporating such addenda in emulsions are relatively simple and wellknown to those skilled in the art of emulsion making.- For example, it is convenient to add them from solutions in appropriate solvents, in which case the solvent selected should be completely free from any deleterious effect on the ultimate lightsensitive material s. Methanol, isopropanol, pyridine, water, etc., alone or in admixtures, have proven satisfactory as solvents for the electron acceptors and halogen conductors. The type of silver halide emulsions that can be sensitized with these dyes include any of those prepared with hydrophilic colloids that are known to be satisfactory for dispersing silver halides, for example, emulsions comprising natural materials such asgelatin, albumin, agar-agar, gum arabic,

'. alginic acid, etc., and hydrophilic synthetic resins such as polyvinyl alcohol, polyvinyl pyrrolidone, cellulose ethers, partially hydrolyzed cellulose acetate, and the like. The binding agents for the emulsion layer can also contain dispersed polymerized vinyl compounds such as disclosed, for example, in US. Pat. Nos. 3,142,568, 3,193,386, 3,062,674 and 3,220,844, and include the water-insoluble polymers of alkyl acrylates and methacrylates, acrylic acid, sulfoalkyl acrylates or methacrylates and the like.

The overlayers are generally very effective in maintaining constant reproducibility of image properties with used developers when silver halide direct-positive emulsions are used in the element wherein a predominantamount of the halide is bromide. The overlayer preferably contains chloride ions or silver chloride, but generally any silver compounds more soluble than silver bromide including silver ferrocyanides can be utilized as they will be displaced by bromide. When the overlayer is a silver compound, it is preferably desensitizedto light. Typical desensitizers useful for this purpose include rhodium ammonium chloride, l-phenyl-- mercaptotetrazole, methyl benzotriazole, 4-nitro-6- chlorobenzotriazole and the like. Useful concentrations of these desensitizers are from about 0.1 to 0.5 per cent per mole of silver.

The overlayersand silver halide layers of this invention can be hardened by any acceptable means known in the photographic art; however, aldehyde hardeners such as formaldehyde and mucochloric acid are preferred. The overlayers can be coated at any suitable concentration to provide the necessary stability in image properties, but they are preferably coated at about 7 to about mg/dm of the ingredient to be reduced by the bromide ions.

This invention can be further illustrated by the following examples.

Example 1 A silver halide direct-positive emulsion is prepared by adding an aqueous solution containing 170 g. of silver nitrate which have been partially reduced to an aqueous solution'containing 131 g. of potassium bromide and 2.25 g. of potassium iodide. The precipitate is decanted, washed, gelatin added, chilled and set. This emulsion has fog specks distributed throughout the grains as a result of the partial reduction of the silver nitrate.

To separate aliquot portions of the emulsion are added 0.127 g. of diphenylamino-5-[(3-ethy1-2(3H)- benzoxasolylidene )-ethylene ]-5-isothiohydantoin in methanol solution. A solution containing 0.0018 g. of potassium chloroaurate, a gelatin solution and a mucochloric acid solution are also added. To one of the portions is added an aqueous solution containing 0.0012 g. of nitron.

The emulsions are coated on a support and dried. Respective samples of the emulsions are then exposed in a sensitometer and developed in a hydroquinone-lphenyl-3-pyrazolidone developer containing sodium bisulfite and methoxy polyethylene glycol.

The samples containing the nitron contain no noticeable yellow physical development fog, while the control sample containing no nitron contains substantial yellow physical development fog.

Samples of the exposed control emulsion and the emulsion containing nitron are then developed in the samples of the above developer which contains respectively 8 g., 14 g., 18 g. and 28 g. ofsodium bromide per liter to approximate conditions of using extensively used developers. When the amount of bromide is increasing, the Dmax of nitron-containing samples is slightly decreasing, whereas in control samples without nitron, Dmax, gamma and yellow fog are highly increasing. v

. It is apparent that the nitron represses the solvent action of extensively used developers which contain higher concentrations of bromide ions.

Similar results are obtained when nitron is used in an overlayer on the emulsion or in a gelatin interlayer within the layer arrangement.

Similar results are also obtained when the dyes diphe- Example 2 A fogged silver halide, direct-positive emulsion containing silver halide grains having silver bromide outer layers or shells is prepared according to Allentoff and Fogler, application Ser. No. 582,262 filed Sept. 27,

1966 now US. Pat. No. 3,477,852, issued Nov. 11,.

1969.-To one portion of the emulsion is added 650 milligrams per mole of silver of nitron. Another portion served as a control.

Samples of the coated emulsion are exposed on a sensitometer and developed for 2% minutes in Kodak D- developer containing 1 g. of hypo per liter. The control sample produces a high degree of yellow fog, whereas the sample containing nitron has no apparent yellow fog.

The background density is measured through a blue filter to give an indication of yellow stain. The control sample has a density of 0.15 while the sample containing 650 mg. of nitron per mole of silver has a density of 0.10.

Example 3 rhodanine sodium salt; and l g. of potassium iodide/- mole of silver halide. Formaldehyde is added as a hardener, and part of the emulsion is coated on a polyethylene terephthalate support. To a second part of the emulsion, 1.5 g. of nitron is added and the emulsion coated on a similar support. Both emulsions are coated at a coverage of 450 mg. of silver and 300 mg. of gelatin/ft? A sample of each coating is exposed on an intensity scale sensitometer and processed for 1% minutes in an amine developer of the type described in Example l of Masseth, U.S. application Ser. No. 661,532 now US. Pat. No. 3,573,914, issued Apr. 6, 1971 (Devel- I oper A), fixed, washed and dried using a roller'transport continuous processing machine. The following results are obtained. 7

A silver chloride emulsion is made bymixing, over a period of 1 minute at 35 C.,'a gelatin solution contain- I ing 100 g. of silver nitrate with an aqueous gelatin solution containing 150 g. of sodium chloride and 0.01 g. of a mixture of rhodium and ammonium chloride. The emulsion is neutralized to a pH of 5.6 and a phthalated gelatin solution is added. The emulsion is coagulated by lowering the pH, then decanted and taken up in a gelatin solution. Mucochloric acid (0.5 g.), nitron (l g.) and l-phenyl--mercaptotetrazole (0.1 g.) are added to the emulsion.

The above chloride'emulsion is thencoated at a coverage of 10 mg. of silver per dm over an X-ray duplicating direct-positive emulsion. which has a gelatin overlayer.

The X-ray duplicating film having a fogged directpositive emulsion is processed in a developer containing various concentrations of bromide ion simulating the build-up of bromide during extensive processing.

Cone. of Br grains which produce a density of at least 0.5 when developed, without exposure, for 5 minutes at 68 F. in Developer A having the composition set forth below, when the emulsion is coated at a silver coverage of 50 mg. to 500 mg. per square foot.

Developer A N-methyl-p-aminophenol sulfate 2.5 g. sodium sulfite (anhydrous) 30.0 g. hydroquinone 2.5 g. sodium metaborate 10.0 g. potassium bromide 0.5 g. water to make 1.0 1.

Although the invention has been described in considerable detail with particular reference to certain preferred embodiments thereof, variations and modifications can be effected within the spirit and scope of the invention.

We claim: 1. An improved process for making direct-positive silver halide grains containing a fogging concentration The maximum image density of the elements having chloride overcoats is much more stable and the relative speed and gamma of the photographic element is much more stable as compared with the element without the chloride overcoat.

Similar results are obtained when silver ferricyanide is used in the overlayer in place of silver chloride.

As used herein, and in the appended claims, fogged" refers to emulsions containing silver halide of metallic silver particles from an aqueous solution of a water-soluble silver salt and an aqueous solution of a water-soluble halide comprising 7 the improvement of contacting the water-soluble silver salt with a reducing agent to form said metallic silver particles before reacting said water-soluble silver salt with said water-soluble halide.

2. A process according to claim 1 wherein said silver salt is contacted with stannous chloride, thiourea dioxide, formalin or an alkaline arsenite reducing agent.

. i 3. A process according to claim 2 wherein said silver halide grains are chemically fogged after formation of said grains.

4. A process according to claim 1 wherein said silver salt is mixed with polyvinyl pyrrolidone before contacting it with said reducing agent. 7

5. In a process for making a fogged, direct-positive photographic silver halide emulsion comprising silver halide grains containing a fogging concentration of metallic silver particles, which process comprises reacting a water-soluble silver salt with a water-soluble halide, the improvement comprising A. contacting said water-soluble silver salt with a reducing agent to form said metallic silver particles before reacting said water-soluble silver salt with said water-soluble halide, and

B. adding nitron to said emulsion.

6. A process as in claim 5 wherein from about mg. to about 2 gms. of nitron per mole of silver is added to said emulsion.

7. A process for making a fogged, direct-positive photographic silver halide emulsion comprising silver halide grains containing a fogging concentration of mewater-soluble halide to produce photographic silver halide, and v C. adding to said emulsion from about 100 mg. to

about 2 gms. of nitron per mole of silver. 

2. A process according to claim 1 wherein said silver salt is contacted with stannous chloride, thiourea dioxide, formalin or an alkaline arsenite reducing agent.
 3. A process according to claim 2 wherein said silver halide grains are chemically fogged after formation of said grains.
 4. A process according to claim 1 wherein said silver salt is mixed with polyvinyl pyrrolidone before contacting it with said reducing agent.
 5. In a process for making a fogged, direct-positive photographic silver halide emulsion comprising silver halide grains containing a fogging concentration of metallic silver particles, which process comprises reacting a water-soluble silver salt with a water-soluble halide, the improvement comprising A. contacting said water-soluble silver salt with a reducing agent to form said metallic silver particles before reacting said water-soluble silver salt with said water-soluble halide, and B. adding nitron to said emulsion.
 6. A process as in claim 5 wherein from about 100 mg. to about 2 gms. of nitron per mole of silver is added to said emulsion.
 7. A process for making a fogged, direct-positive photographic silver halide emulsion comprising silver halide grains containing a fogging concentration of metallic silver particles, said process comprising A. partially reducing a water-soluble silver salt by contacting it with a reducing agent whereby said metallic silver particles are formed, B. then reacting the water-soluble silver salt with a water-soluble halide to produce photographic silver halide, and C. adding to said emulsion from about 100 mg. to about 2 gms. of nitron per mole of silver. 